Cranial perforators are special purpose drills which are used to bore holes through the skull during cranial surgery. Such holes may be needed to vent fluids from the region surrounding the brain, to provide small passageways to the brain for the insertion and removal of instruments, or to position a cranial saw for subsequent use in removing a larger piece of the skull.
Regardless of the end use of the hole being made, it is critical that the cranial perforator cease its boring action as quickly as possible after it passes through the skull and, if possible, before it encounters, and thereby damages, the delicate dura tissue surrounding the brain, or the brain itself. To this end, cranial perforators have traditionally utilized a special "safety construction" designed to permit forward penetration by the perforator only so long as the perforator's leading tip is encountering bone, and to halt forward penetration by the perforator as quickly as possible after the perforator's leading tip passes through the bone and, hopefully, before it encounters the soft tissue beneath the bone. More particularly, cranial perforators have traditionally comprised a drill head assembly having a pair of drills disposed in concentric relation to one another, with the inner drill leading the outer drill so that a bore-counterbore opening is formed as the perforator penetrates into the skull. The two drills are coupled to a rear support and drive assembly via a special drive mechanism such that (1) both drills are rotatably driven so long as the leading inner drill is encountering a resistive surface (i.e., bone) and (2) both drills are no longer rotatably driven when the inner drill stops encountering the resistive surface (i.e., when the inner drill passes through the bone) while the outer drill is still encountering the resistive surface (i.e., while it is still cutting through the bone). Inasmuch as the leading inner drill and the trailing outer drill are adapted to cut in a bore-counterbore arrangement, the shoulder of bone formed at the intersection of the bore-counterbore opening automatically impedes further progress of the perforator toward the brain once the inner and outer drills are no longer rotating. As a result, the surgeon using the cranial perforator does not have to concentrate on the amount of pressure to be applied to the cranial perforator as the remaining bone becomes thinner and thinner, and generally need not fear that the perforator will plunge through the bone into the head so as to severely damage the delicate dura tissue or the brain itself. Such cranial perforators have included both reusable, disposable and disposable tip models.
U.S. Pat. No. 4,600,006, issued Jul. 15, 1986 to John W. Baker for "Cranial Perforator", discloses an improved form of cranial perforator incorporating the foregoing "safety construction". Still other cranial perforators incorporating the foregoing "safety construction" are disclosed in U.S. Pat. No. 4,803,982, issued Feb. 14, 1989 to John W. Baker for "Cranial Perforator", pending U.S. patent application Ser. No. 07/423,660, filed Oct. 18, 1989, U.S. Pat. No. 4,884,571, issued Dec. 5, 1989 to John W. Baker for "Cranial Perforator With Reentrant Cutting Segment", and pending U.S. patent application Ser. No. 07/299,084, filed Jan. 23 1989.
As noted above, the "safety construction" of the foregoing perforators is characterized by inner and outer drills which are coupled to a rear support and drive assembly via a special drive mechanism such that both drills are rotatably driven so long as the leading inner drill is encountering a resistive surface (i.e., bone) and both drills are no longer rotatably driven when the inner drill stops encountering the resistive surface (i.e., when the inner drill passes through the bone) while the outer drill is still encountering the resistive surface (i.e., while it is still cutting through the bone). However, it has been observed that even with these cranial perforators, the inner drill continues to rotate some angular amount after the inner drill has penetrated the skull.
A portion of the aforementioned post-penetration angular rotation of the inner drill is associated with the design of the special drive mechanism employed in the perforators, which consists of cams and cam followers which withdraw engaging portions of the inner drill forward out of driving engagement with the rear support and drive assembly. Details on the design and function of this special drive mechanism are provided in U.S. Pat. No. 4,600,006, which is incorporated herein by reference. For convenience, the portion of the aforementioned post-penetration angular rotation of the inner drill associated with the disengagement of the inner drill from the rear support and drive assembly will be referred to as "disengagement rotation".
Another portion of the aforementioned post-penetration angular rotation of the inner drill is associated with the angular momentum resident in the quickly rotating inner drill at the moment the inner drill disengages from the rear support and drive assembly. In effect, the angular momentum stored in the quickly rotating inner drill at the moment of full disengagement causes the inner drill to continue rotating until this angular momentum is dissipated through friction. For convenience, this portion of the aforementioned post-penetration angular rotation of the inner drill will be referred to as "momentum rotation".
The aforementioned post-penetration "disengagement rotation" is believed to be an unavoidable consequence of the special drive mechanism associated with the aforementioned cranial perforators, whereas the aforementioned post-penetration "momentum rotation" is not. Since any rotation of the inner drill after it passes through the bone increases the possibility of damaging the dura or the brain, it is desirable to stop the rotation of the inner drill as quickly as possible after the inner drill penetrates the skull.